A method for welding metal members together is known as friction stir welding (FSW). Friction stir welding is a method utilizing friction heat to join members together by heating and softening and also inducing a plasticizing flow by the rotation of a rotation tool (hereafter called FSW tool) by insertion while pressing the rotation tool up against the boundary surface of the metal members to be joined together.
A manufacturing method for a water-cooling jacket comprised of a jacket piece including a cavity with an opening in one section and a sealer to seal the opening of the cavity is disclosed in patent literature 1. In this manufacturing method a butt joint is formed by a side surface on a step formed in the opening of the jacket piece and the side surface of the sealant, and welding the abutting surfaces of the butt joint by friction welding stirring.